The present claimed invention relates to the field of digital communication. Specifically, the present claimed invention relates to an apparatus and a method for determining frame rate of a data frame in a communication system by using apriori knowledge of the data in a frame.
Wireless telephony, e.g. cellular phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technology. Because of increasing demand and limited resources, a need arises to improve their fidelity and performance.
Referring to prior art FIG. 1A, a conventional base station 104, e.g. cell, and a mobile unit 102, e.g. a cell phone, are shown. A CDMA system uses a common bandwidth to transmit the pilot signal and a data signal 106 between a base station 104 and a mobile unit 102, for multiple users. Hence, the bandwidth is occupied by an combination of many signals. Variable rate communication systems transmit data in units called data frames, and, as its name indicates, at various frame rates.
In the case of CDMA systems, frame rate information is not transmitted from one unit to the other, e.g. from the originating base station to the receiver. Thus, the receiver needs to determine the frame rate of any and all data frames received from the transmitter on a frame-by-frame basis. That is, each successive data frame can have a different frame rate, depending upon which frame rate is most efficient given the amount of data in the signal at that time. As such, the receiver needs to have a method for determining the frame rate at which a given data frame is being transmitted, when the data frame is received at the unit.
Conventional methods used to determine the frame rate of a data frame exist. However, these conventional methods utilize a characteristic, or property, of the signal to attempt to identify the true frame rate of the data frame. For example, one prior art method evaluates the intensity of the signal. More specifically, a full-rate transmission of a data frame will have a higher intensity level because it packs a higher quantity of bits in the data frame, while an eighth-rate transmission of a data frame will have a lower intensity level, because it packs a lower quantity of bits in the data frame. As mentioned, because the frame rate of a data frame is not directly indicated by the transmitter, any conventional method of determining the frame rate can only provide a level of confidence that the guessed frame rate is the true frame rate of the data frame. Under different circumstances, each conventional method of determining frame rates may have weaknesses and strengths. If additional methods, using different strategies, for determining frame rates are available, a more reliable level of confidence of the frame rate can be established. Hence a need arises for additional methods, and novel strategies, for determining the frame rate of a data frame.
Additionally, a method and device for determining the frame rate of a data frame in a high noise level, variable rate communication system must not require significant revamping of the existing communication system. That is, in implementing a viable method and device for frame rate determination in a variable rate communication system, components that are well known in the art, and are compatible with existing communication systems, are necessary if cost and reliability are to be optimized. In so doing, the need to incur costly expenditures for retrofitting existing communication systems or for building custom components is avoided. However, if modification is required for implementing some methods of determining a frame rate of a data frame, a need arises to keep these modifications to a minimum.
In summary, a need exists for a method and device for improving the fidelity and performance of a CDMA communication system. In particular, a need arises for frame rate determination in a variable rate communication system. More specifically, a need arises for additional methods, that use new strategies, for determining the frame rate. Still another need exists for a method and system which meets both of the above cited needs and wherein the method and device is easy to implement and is conducive to use with existing variable rate communication systems. Lastly, if modification is required for implementing some methods of determining a frame rate of a data frame, a need arises to keep these modifications to a minimum.
The present invention provides a method and apparatus for improving the fidelity and performance of digital communication. In particular, the present invention provides a method and apparatus for determining the frame rate of a data frame in a variable rate communication system. More specifically, the present invention determines the frame rate using new methods, or algorithms. Additionally, the method and apparatus of the present invention is easy to implement and is conducive to use with existing variable rate communication systems. Lastly, the present invention provides some methods of determining a frame rate of a data frame that require only minor modifications to the communication device hardware.
Specifically, the present invention utilizes apriori knowledge of a logic level for a portion of a data frame to determine the frame rate of the data frame. One embodiment utilizes the fact that, for convolutionally encoded data that uses tail bits, each data frame uses a tail bit portion to reset the shift registers used for encoding the data frame. The tail bit portion of the data frame is established as having eight bits with a low logic level, e.g. xe2x80x9c0,xe2x80x9d for the case of constraint length K=9, for a convolutional encoder, though other constraint lengths are possible. However, with different frame rates in a variable rate communication system, the last eight bits span different amounts of time. This provides a useful discriminator between the different possible frame rates as applied to an actual data frame. This information generates, in one embodiment, a method to enhance the level of reliability in determining a frame rate for a data frame. In particular, if a correlation result between a received data signal and its apriori-established transmitted data signal exceeds a threshold value, then a good level of confidence can be established that the assumed frame rate is probably correct.
In one embodiment, the apriori knowledge that the tail bits have a logic zero level can be used to evaluate the frame rate in a different manner. In this embodiment, performance of the Viterbi decoder is used to determine the frame rate of the data frame. Specifically, if the chosen frame rate implemented by a Viterbi decoder yields a state change, in an apriori-established direction or sequence, for the last eight bits of the data frame, for at least one possible path in the trellis diagram evaluation, then a good level of confidence exists that the frame rate implemented is the true frame rate of the data frame. In addition, a branch metric is calculated for an expected input of 0 and an expected input of 1 for every state. The xe2x80x9cexpected 0xe2x80x9d branch metrics are summed together for all states of a given time stage in the tail-bit portion of the trellis diagram. Similarly, the xe2x80x9cexpected 1xe2x80x9d branch metrics are also summed together for all states of a given time stage. A delta is then calculated from the difference between the summed expected 1 branch metric and the summed expected 0 branch metric for each given time stage. Next, the deltas for all time stages in the tail bit portion are summed to obtain a deltasum. If the deltasum is a large positive value, then a good level of confidence exists that the frame rate utilized by the Viterbi decoder is the correct one. If the deltasum is a smaller number, then a lower level of confidence exists that the frame rate utilized by the Viterbi decoder is the correct one. The branch metric deltas for the tail bits, provided by the symbol detector portion of the Viterbi decoder, provide a soft decision about the frame rate. In the present embodiment, a lower metric, and hence a larger deltasum, is established as representing a higher level of confidence. However, the present invention is well-suited to an alternative embodiment where a higher metric, and hence a smaller deltasum, is established as representing a higher level of confidence.
In another embodiment, the normal xe2x80x9chard decisionxe2x80x9d decoded tail bit outputs of the Viterbi Decoder are used along with xe2x80x9csoft decisionsxe2x80x9d that arise from the branch metric deltas for the tail bits described in the previous embodiment. The hard decision decoded tail bit outputs are provided by a traceback operation implemented by the sequence detector portion of the Viterbi decoder. Hence, the present embodiment utilizes both the traceback operation and the symbol detector portion of the Viterbi decoder. Consequently, a more reliable soft output is obtained, thereby enhancing the reliability of the frame rate determination process.
Specifically, the second embodiment computes and stores the eight soft decisions, e.g. the branch metric deltas, for each of the eight tail bit time stages, e.g. delta(1) through delta(8), as referred to in the previous embodiment. In addition, the normal decoding operation also occurs. Once the eight hard decisions are completed, the eight soft symbols are computed by combining the hard decisions with the soft decisions. Next, each of the eight soft symbols are correlated to the expected tail bits to obtain a correlation result. The correlation result essentially provides an indication of whether the received bit was a 0, as expected for the tail bit, and provides a level of confidence for the indication. For each frame rate, a correlation result is determined and compared with a respective threshold value for each frame rate. A favorable comparison indicates that the chosen frame rate is the correct one.
One embodiment of the present invention provides an algorithm for determining the frame rate of a data portion of a data frame with no hardware changes to the communication device. In another embodiment, minor hardware changes to the communication device are required for a different algorithm for determining the frame rate of a data portion of a data frame.
These and other objects and advantages of the present invention will become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.